[pve-devel] [POC storage v6 16/37] add backup provider example

[Fiona Ebner <f.ebner@proxmox.com>]

The example uses a simple directory structure to save the backups,
grouped by guest ID. VM backups are saved as configuration files and
qcow2 images, with backing files when doing incremental backups.
Container backups are saved as configuration files and a tar file or
squashfs image (added to test the 'directory' restore mechanism).

Whether to use incremental VM backups and which backup mechanisms to
use can be configured in the storage configuration.

The 'nbdinfo' binary from the 'libnbd-bin' package is required for
backup mechanism 'nbd' for VM backups, the 'mksquashfs' binary from the
'squashfs-tools' package is required for backup mechanism 'squashfs' for
containers.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---

Changes in v6:
* Remove superfluous $storeid parameter from methods. It's already
  part of $self.
* Rename restore_get_*_config() methods to archive_get_*_config().
* Replace job/backup hooks with dedicated methods.
* Remove backup_get_archive_name() method. The archive name is now
  returned as part of the backup_init() method.
* Supersede backup_get_task_size() method and return
  extensible-in-the-future stats as part of the backup_cleanup()
  method.

 .../BackupProvider/Plugin/DirectoryExample.pm | 778 ++++++++++++++++++
 src/PVE/BackupProvider/Plugin/Makefile        |   2 +-
 .../Custom/BackupProviderDirExamplePlugin.pm  | 308 +++++++
 src/PVE/Storage/Custom/Makefile               |   5 +
 src/PVE/Storage/Makefile                      |   1 +
 5 files changed, 1093 insertions(+), 1 deletion(-)
 create mode 100644 src/PVE/BackupProvider/Plugin/DirectoryExample.pm
 create mode 100644 src/PVE/Storage/Custom/BackupProviderDirExamplePlugin.pm
 create mode 100644 src/PVE/Storage/Custom/Makefile

diff --git a/src/PVE/BackupProvider/Plugin/DirectoryExample.pm b/src/PVE/BackupProvider/Plugin/DirectoryExample.pm
new file mode 100644
index 0000000..4d90c58
--- /dev/null
+++ b/src/PVE/BackupProvider/Plugin/DirectoryExample.pm
@@ -0,0 +1,778 @@
+package PVE::BackupProvider::Plugin::DirectoryExample;
+
+use strict;
+use warnings;
+
+use Fcntl qw(SEEK_SET);
+use File::Path qw(make_path remove_tree);
+use IO::File;
+use IPC::Open3;
+
+use PVE::Storage::Common;
+use PVE::Storage::Plugin;
+use PVE::Tools qw(file_get_contents file_read_firstline file_set_contents run_command);
+
+use base qw(PVE::BackupProvider::Plugin::Base);
+
+# Private helpers
+
+my sub log_info {
+    my ($self, $message) = @_;
+
+    $self->{'log-function'}->('info', $message);
+}
+
+my sub log_warning {
+    my ($self, $message) = @_;
+
+    $self->{'log-function'}->('warn', $message);
+}
+
+my sub log_error {
+    my ($self, $message) = @_;
+
+    $self->{'log-function'}->('err', $message);
+}
+
+# Try to use the same bitmap ID as last time for incremental backup if the storage is configured for
+# incremental VM backup. Need to start fresh if there is no previous ID or the associated backup
+# doesn't exist.
+my sub get_bitmap_id {
+    my ($self, $vmid, $vmtype) = @_;
+
+    return if $self->{'storage-plugin'}->get_vm_backup_mode($self->{scfg}) ne 'incremental';
+
+    my $previous_info_dir = "$self->{scfg}->{path}/$vmid/";
+
+    my $previous_info_file = "$previous_info_dir/previous-info";
+    my $info = file_read_firstline($previous_info_file) // '';
+    $self->{$vmid}->{'old-previous-info'} = $info;
+    my ($bitmap_id, $previous_backup_id) = $info =~ m/^(\d+)\s+(\d+)$/;
+    my $previous_backup_dir =
+	$previous_backup_id ? "$self->{scfg}->{path}/$vmid/$vmtype-$previous_backup_id" : undef;
+
+    if ($bitmap_id && -d $previous_backup_dir) {
+	$self->{$vmid}->{'previous-backup-dir'} = $previous_backup_dir;
+    } else {
+	# need to start fresh if there is no previous ID or the associated backup doesn't exist
+	$bitmap_id = $self->{$vmid}->{'backup-time'};
+    }
+
+    $self->{$vmid}->{'bitmap-id'} = $bitmap_id;
+    make_path($previous_info_dir);
+    die "unable to create directory $previous_info_dir\n" if !-d $previous_info_dir;
+    file_set_contents($previous_info_file, "$bitmap_id $self->{$vmid}->{'backup-time'}");
+
+    return $bitmap_id;
+}
+
+# NOTE: This is just for proof-of-concept testing! A backup provider plugin should either use the
+# 'nbd' backup mechansim and use the NBD protocol or use the 'file-handle' mechanism. There should
+# be no need to use /dev/nbdX nodes for proper plugins.
+my sub bind_next_free_dev_nbd_node {
+    my ($options) = @_;
+
+    # /dev/nbdX devices are reserved in a file. Those reservations expires after $expiretime.
+    # This avoids race conditions between allocation and use.
+
+    die "file '/sys/module/nbd' does not exist - 'nbd' kernel module not loaded?"
+	if !-e "/sys/module/nbd";
+
+    my $line = PVE::Tools::file_read_firstline("/sys/module/nbd/parameters/nbds_max")
+	or die "could not read 'nbds_max' parameter file for 'nbd' kernel module\n";
+    my ($nbds_max) = ($line =~ m/(\d+)/)
+	or die "could not determine 'nbds_max' parameter for 'nbd' kernel module\n";
+
+    my $filename = "/run/qemu-server/reserved-dev-nbd-nodes";
+
+    my $code = sub {
+	my $expiretime = 60;
+	my $ctime = time();
+
+	my $used = {};
+	my $latest = [0, 0];
+
+	if (my $fh = IO::File->new ($filename, "r")) {
+	    while (my $line = <$fh>) {
+		if ($line =~ m/^(\d+)\s(\d+)$/) {
+		    my ($n, $timestamp) = ($1, $2);
+
+		    $latest = [$n, $timestamp] if $latest->[1] <= $timestamp;
+
+		    if (($timestamp + $expiretime) > $ctime) {
+			$used->{$n} = $timestamp; # not expired
+		    }
+		}
+	    }
+	}
+
+	my $new_n;
+	for (my $count = 0; $count < $nbds_max; $count++) {
+	    my $n = ($latest->[0] + $count) % $nbds_max;
+	    my $block_device = "/dev/nbd${n}";
+	    next if $used->{$n}; # reserved
+	    next if !-e $block_device;
+
+	    my $st = File::stat::stat("/run/lock/qemu-nbd-nbd${n}");
+	    next if defined($st) && S_ISSOCK($st->mode) && $st->uid == 0; # in use
+
+	    # Used to avoid looping if there are other issues then the NBD node being in use
+	    my $socket_error = 0;
+	    eval {
+		my $errfunc = sub {
+		    my ($line) = @_;
+		    $socket_error = 1 if $line =~ m/^qemu-nbd: Failed to set NBD socket$/;
+		    log_warn($line);
+		};
+		run_command(["qemu-nbd", "-c", $block_device, $options->@*], errfunc => $errfunc);
+	    };
+	    if (my $err = $@) {
+		die $err if !$socket_error;
+		log_warn("unable to bind $block_device - trying next one");
+		next;
+	    }
+	    $used->{$n} = $ctime;
+	    $new_n = $n;
+	    last;
+	}
+
+	my $data = "";
+	$data .= "$_ $used->{$_}\n" for keys $used->%*;
+
+	PVE::Tools::file_set_contents($filename, $data);
+
+	return defined($new_n) ? "/dev/nbd${new_n}" : undef;
+    };
+
+    my $block_device =
+	PVE::Tools::lock_file('/run/lock/qemu-server/reserved-dev-nbd-nodes.lock', 10, $code);
+    die $@ if $@;
+
+    die "unable to find free /dev/nbdX block device node\n" if !$block_device;
+
+    return $block_device;
+}
+
+# Backup Provider API
+
+sub new {
+    my ($class, $storage_plugin, $scfg, $storeid, $log_function) = @_;
+
+    my $self = bless {
+	scfg => $scfg,
+	storeid => $storeid,
+	'storage-plugin' => $storage_plugin,
+	'log-function' => $log_function,
+    }, $class;
+
+    return $self;
+}
+
+sub provider_name {
+    my ($self) = @_;
+
+    return 'dir provider example';
+}
+
+# Hooks
+
+sub job_init {
+    my ($self, $start_time) = @_;
+
+    log_info($self, "job init called");
+
+    if (!-e '/sys/module/nbd/parameters') {
+	die "required 'nbd' kernel module not loaded - use 'modprobe nbd nbds_max=128' to load it"
+	    ." manually\n";
+    }
+
+    log_info($self, "backup provider initialized successfully for new job $start_time");
+
+    return;
+}
+
+sub job_cleanup {
+    my ($self) = @_;
+
+    log_info($self, "job cleanup called");
+
+    return;
+}
+
+sub backup_init {
+    my ($self, $vmid, $vmtype, $backup_time) = @_;
+
+    my $archive = "${vmid}/${vmtype}-${backup_time}";
+
+    log_info($self, "backup start hook called");
+
+    my $backup_dir = $self->{scfg}->{path} . "/" . $archive;
+
+    make_path($backup_dir);
+    die "unable to create directory $backup_dir\n" if !-d $backup_dir;
+
+    $self->{$vmid}->{'backup-time'} = $backup_time;
+    $self->{$vmid}->{'backup-dir'} = $backup_dir;
+
+    $self->{$vmid}->{archive} = $archive;
+    return { 'archive-name' => $archive };
+}
+
+sub backup_cleanup {
+    my ($self, $vmid, $vmtype, $success, $info) = @_;
+
+    if ($success) {
+	log_info($self, "backup cleanup called - success");
+	my $size = 0;
+	my $backup_dir = $self->{$vmid}->{'backup-dir'};
+	my @backup_files = glob("$backup_dir/*");
+	$size += -s $_ for @backup_files;
+	my $stats = { 'archive-size' => $size };
+	return { 'stats' => $stats };
+    } else {
+	log_info($self, "backup cleanup called - failure");
+
+	$self->{$vmid}->{failed} = 1;
+
+	if (my $dir = $self->{$vmid}->{'backup-dir'}) {
+	    eval { remove_tree($dir) };
+	    log_warning($self, "unable to clean up $dir - $@") if $@;
+	}
+
+	# Restore old previous-info so next attempt can re-use bitmap again
+	if (my $info = $self->{$vmid}->{'old-previous-info'}) {
+	    my $previous_info_dir = "$self->{scfg}->{path}/$vmid/";
+	    my $previous_info_file = "$previous_info_dir/previous-info";
+	    file_set_contents($previous_info_file, $info);
+	}
+    }
+}
+
+sub backup_container_prepare {
+    my ($self, $vmid, $info) = @_;
+
+    my $dir = $self->{$vmid}->{'backup-dir'};
+    chown($info->{'backup-user-id'}, -1, $dir) or die "unable to change owner for $dir\n";
+
+    return;
+}
+
+sub backup_get_mechanism {
+    my ($self, $vmid, $vmtype) = @_;
+
+    return ('directory', undef) if $vmtype eq 'lxc';
+
+    if ($vmtype eq 'qemu') {
+	my $backup_mechanism = $self->{'storage-plugin'}->get_vm_backup_mechanism($self->{scfg});
+	return ($backup_mechanism, get_bitmap_id($self, $vmid, $vmtype));
+    }
+
+    die "unsupported guest type '$vmtype'\n";
+}
+
+sub backup_handle_log_file {
+    my ($self, $vmid, $filename) = @_;
+
+    my $log_dir = $self->{$vmid}->{'backup-dir'};
+    if ($self->{$vmid}->{failed}) {
+	$log_dir .= ".failed";
+    }
+    make_path($log_dir);
+    die "unable to create directory $log_dir\n" if !-d $log_dir;
+
+    my $data = file_get_contents($filename);
+    my $target = "${log_dir}/backup.log";
+    file_set_contents($target, $data);
+}
+
+my sub backup_file {
+    my ($self, $vmid, $device_name, $size, $in_fh, $bitmap_mode, $next_dirty_region, $bandwidth_limit) = @_;
+
+    # TODO honor bandwidth_limit
+
+    my $previous_backup_dir = $self->{$vmid}->{'previous-backup-dir'};
+    my $incremental = $previous_backup_dir && $bitmap_mode eq 'reuse';
+    my $target = "$self->{$vmid}->{'backup-dir'}/${device_name}.qcow2";
+    my $target_base = $incremental ? "${previous_backup_dir}/${device_name}.qcow2" : undef;
+    my $create_cmd = ["qemu-img", "create", "-f", "qcow2", $target, $size];
+    push $create_cmd->@*, "-b", $target_base, "-F", "qcow2" if $target_base;
+    run_command($create_cmd);
+
+    my $nbd_node;
+    eval {
+	# allows to easily write to qcow2 target
+	$nbd_node = bind_next_free_dev_nbd_node([$target, '--format=qcow2']);
+	# FIXME use nbdfuse like in qemu-server rather than qemu-nbd. Seems like there is a race and
+	# sysseek() can fail with "Invalid argument" if done too early...
+	sleep 1;
+
+	my $block_size = 4 * 1024 * 1024; # 4 MiB
+
+	my $out_fh = IO::File->new($nbd_node, "r+")
+	    or die "unable to open NBD backup target - $!\n";
+
+	my $buffer = '';
+	my $skip_discard;
+
+	while (scalar((my $region_offset, my $region_length) = $next_dirty_region->())) {
+	    sysseek($in_fh, $region_offset, SEEK_SET)
+		// die "unable to seek '$region_offset' in NBD backup source - $!\n";
+	    sysseek($out_fh, $region_offset, SEEK_SET)
+		// die "unable to seek '$region_offset' in NBD backup target - $!\n";
+
+	    my $local_offset = 0; # within the region
+	    while ($local_offset < $region_length) {
+		my $remaining = $region_length - $local_offset;
+		my $request_size = $remaining < $block_size ? $remaining : $block_size;
+		my $offset = $region_offset + $local_offset;
+
+		my $read = sysread($in_fh, $buffer, $request_size);
+		die "failed to read from backup source - $!\n" if !defined($read);
+		die "premature EOF while reading backup source\n" if $read == 0;
+
+		my $written = 0;
+		while ($written < $read) {
+		    my $res = syswrite($out_fh, $buffer, $request_size - $written, $written);
+		    die "failed to write to backup target - $!\n" if !defined($res);
+		    die "unable to progress writing to backup target\n" if $res == 0;
+		    $written += $res;
+		}
+
+		if (!$skip_discard) {
+		    eval { PVE::Storage::Common::deallocate($in_fh, $offset, $request_size); };
+		    if (my $err = $@) {
+			# Just assume that if one request didn't work, others won't either.
+			log_warning(
+			    $self, "discard source failed (skipping further discards) - $err");
+			$skip_discard = 1;
+		     }
+		 }
+
+		$local_offset += $request_size;
+	    }
+	}
+	$out_fh->sync();
+    };
+    my $err = $@;
+
+    eval { run_command(['qemu-nbd', '-d', $nbd_node ]); };
+    log_warning($self, "unable to disconnect NBD backup target - $@") if $@;
+
+    die $err if $err;
+}
+
+my sub backup_nbd {
+    my ($self, $vmid, $device_name, $size, $nbd_path, $bitmap_mode, $bitmap_name, $bandwidth_limit) = @_;
+
+    # TODO honor bandwidth_limit
+
+    die "need 'nbdinfo' binary from package libnbd-bin\n" if !-e "/usr/bin/nbdinfo";
+
+    my $nbd_info_uri = "nbd+unix:///${device_name}?socket=${nbd_path}";
+    my $qemu_nbd_uri = "nbd:unix:${nbd_path}:exportname=${device_name}";
+
+    my $cpid;
+    my $error_fh;
+    my $next_dirty_region;
+
+    # If there is no dirty bitmap, it can be treated as if there's a full dirty one. The output of
+    # nbdinfo is a list of tuples with offset, length, type, description. The first bit of 'type' is
+    # set when the bitmap is dirty, see QEMU's docs/interop/nbd.txt
+    my $dirty_bitmap = [];
+    if ($bitmap_mode ne 'none') {
+	my $input = IO::File->new();
+	my $info = IO::File->new();
+	$error_fh = IO::File->new();
+	my $nbdinfo_cmd = ["nbdinfo", $nbd_info_uri, "--map=qemu:dirty-bitmap:${bitmap_name}"];
+	$cpid = open3($input, $info, $error_fh, $nbdinfo_cmd->@*)
+	    or die "failed to spawn nbdinfo child - $!\n";
+
+	$next_dirty_region = sub {
+	    my ($offset, $length, $type);
+	    do {
+		my $line = <$info>;
+		return if !$line;
+		die "unexpected output from nbdinfo - $line\n"
+		    if $line !~ m/^\s*(\d+)\s*(\d+)\s*(\d+)/; # also untaints
+		($offset, $length, $type) = ($1, $2, $3);
+	    } while (($type & 0x1) == 0); # not dirty
+	    return ($offset, $length);
+	};
+    } else {
+	my $done = 0;
+	$next_dirty_region = sub {
+	    return if $done;
+	    $done = 1;
+	    return (0, $size);
+	};
+    }
+
+    my $nbd_node;
+    eval {
+	$nbd_node = bind_next_free_dev_nbd_node([$qemu_nbd_uri, "--format=raw", "--discard=on"]);
+
+	my $in_fh = IO::File->new($nbd_node, 'r+')
+	    or die "unable to open NBD backup source '$nbd_node' - $!\n";
+
+	backup_file(
+	    $self,
+	    $vmid,
+	    $device_name,
+	    $size,
+	    $in_fh,
+	    $bitmap_mode,
+	    $next_dirty_region,
+	    $bandwidth_limit,
+	);
+    };
+    my $err = $@;
+
+    eval { run_command(["qemu-nbd", "-d", $nbd_node ]); };
+    log_warning($self, "unable to disconnect NBD backup source - $@") if $@;
+
+    if ($cpid) {
+	my $waited;
+	my $wait_limit = 5;
+	for ($waited = 0; $waited < $wait_limit && waitpid($cpid, POSIX::WNOHANG) == 0; $waited++) {
+	    kill 15, $cpid if $waited == 0;
+	    sleep 1;
+	}
+	if ($waited == $wait_limit) {
+	    kill 9, $cpid;
+	    sleep 1;
+	    log_warning($self, "unable to collect nbdinfo child process")
+		if waitpid($cpid, POSIX::WNOHANG) == 0;
+	}
+    }
+
+    die $err if $err;
+}
+
+my sub backup_vm_volume {
+    my ($self, $vmid, $device_name, $info, $bandwidth_limit) = @_;
+
+    my $backup_mechanism = $self->{'storage-plugin'}->get_vm_backup_mechanism($self->{scfg});
+
+    if ($backup_mechanism eq 'nbd') {
+	backup_nbd(
+	    $self,
+	    $vmid,
+	    $device_name,
+	    $info->{size},
+	    $info->{'nbd-path'},
+	    $info->{'bitmap-mode'},
+	    $info->{'bitmap-name'},
+	    $bandwidth_limit,
+	);
+    } elsif ($backup_mechanism eq 'file-handle') {
+	backup_file(
+	    $self,
+	    $vmid,
+	    $device_name,
+	    $info->{size},
+	    $info->{'file-handle'},
+	    $info->{'bitmap-mode'},
+	    $info->{'next-dirty-region'},
+	    $bandwidth_limit,
+	);
+    } else {
+	die "internal error - unknown VM backup mechansim '$backup_mechanism'\n";
+    }
+}
+
+sub backup_vm {
+    my ($self, $vmid, $guest_config, $volumes, $info) = @_;
+
+    my $target = "$self->{$vmid}->{'backup-dir'}/guest.conf";
+    file_set_contents($target, $guest_config);
+
+    if (my $firewall_config = $info->{'firewall-config'}) {
+	$target = "$self->{$vmid}->{'backup-dir'}/firewall.conf";
+	file_set_contents($target, $firewall_config);
+    }
+
+    for my $device_name (sort keys $volumes->%*) {
+	backup_vm_volume(
+	    $self, $vmid, $device_name, $volumes->{$device_name}, $info->{'bandwidth-limit'});
+    }
+}
+
+my sub backup_directory_tar {
+    my ($self, $vmid, $directory, $exclude_patterns, $sources, $bandwidth_limit) = @_;
+
+    # essentially copied from PVE/VZDump/LXC.pm' archive()
+
+    # copied from PVE::Storage::Plugin::COMMON_TAR_FLAGS
+    my @tar_flags = qw(
+	--one-file-system
+	-p --sparse --numeric-owner --acls
+	--xattrs --xattrs-include=user.* --xattrs-include=security.capability
+	--warning=no-file-ignored --warning=no-xattr-write
+    );
+
+    my $tar = ['tar', 'cpf', '-', '--totals', @tar_flags];
+
+    push @$tar, "--directory=$directory";
+
+    my @exclude_no_anchored = ();
+    my @exclude_anchored = ();
+    for my $pattern ($exclude_patterns->@*) {
+	if ($pattern !~ m|^/|) {
+	    push @exclude_no_anchored, $pattern;
+	} else {
+	    push @exclude_anchored, $pattern;
+	}
+    }
+
+    push @$tar, '--no-anchored';
+    push @$tar, '--exclude=lost+found';
+    push @$tar, map { "--exclude=$_" } @exclude_no_anchored;
+
+    push @$tar, '--anchored';
+    push @$tar, map { "--exclude=.$_" } @exclude_anchored;
+
+    push @$tar, $sources->@*;
+
+    my $cmd = [ $tar ];
+
+    push @$cmd, [ 'cstream', '-t', $bandwidth_limit * 1024 ] if $bandwidth_limit;
+
+    my $target = "$self->{$vmid}->{'backup-dir'}/archive.tar";
+    push @{$cmd->[-1]}, \(">" . PVE::Tools::shellquote($target));
+
+    my $logfunc = sub {
+	my $line = shift;
+	log_info($self, "tar: $line");
+    };
+
+    PVE::Tools::run_command($cmd, logfunc => $logfunc);
+
+    return;
+};
+
+# NOTE This only serves as an example to illustrate the 'directory' restore mechanism. It is not
+# fleshed out properly, e.g. I didn't check if exclusion is compatible with
+# proxmox-backup-client/rsync or xattrs/ACL/etc. work as expected!
+my sub backup_directory_squashfs {
+    my ($self, $vmid, $directory, $exclude_patterns, $bandwidth_limit) = @_;
+
+    my $target = "$self->{$vmid}->{'backup-dir'}/archive.sqfs";
+
+    my $mksquashfs = ['mksquashfs', $directory, $target, '-quiet', '-no-progress'];
+
+    push $mksquashfs->@*, '-wildcards';
+
+    for my $pattern ($exclude_patterns->@*) {
+	if ($pattern !~ m|^/|) { # non-anchored
+	    push $mksquashfs->@*, '-e', "... $pattern";
+	} else { # anchored
+	    push $mksquashfs->@*, '-e', substr($pattern, 1); # need to strip leading slash
+	}
+    }
+
+    my $cmd = [ $mksquashfs ];
+
+    push @$cmd, [ 'cstream', '-t', $bandwidth_limit * 1024 ] if $bandwidth_limit;
+
+    my $logfunc = sub {
+	my $line = shift;
+	log_info($self, "mksquashfs: $line");
+    };
+
+    PVE::Tools::run_command($cmd, logfunc => $logfunc);
+
+    return;
+};
+
+sub backup_container {
+    my ($self, $vmid, $guest_config, $exclude_patterns, $info) = @_;
+
+    my $target = "$self->{$vmid}->{'backup-dir'}/guest.conf";
+    file_set_contents($target, $guest_config);
+
+    if (my $firewall_config = $info->{'firewall-config'}) {
+	$target = "$self->{$vmid}->{'backup-dir'}/firewall.conf";
+	file_set_contents($target, $firewall_config);
+    }
+
+    my $backup_mode = $self->{'storage-plugin'}->get_lxc_backup_mode($self->{scfg});
+    if ($backup_mode eq 'tar') {
+	backup_directory_tar(
+	    $self,
+	    $vmid,
+	    $info->{directory},
+	    $exclude_patterns,
+	    $info->{sources},
+	    $info->{'bandwidth-limit'},
+	);
+    } elsif ($backup_mode eq 'squashfs') {
+	backup_directory_squashfs(
+	    $self,
+	    $vmid,
+	    $info->{directory},
+	    $exclude_patterns,
+	    $info->{'bandwidth-limit'},
+	);
+    } else {
+	die "got unexpected backup mode '$backup_mode' from storage plugin\n";
+    }
+}
+
+# Restore API
+
+sub restore_get_mechanism {
+    my ($self, $volname) = @_;
+
+    my (undef, $relative_backup_dir) = $self->{'storage-plugin'}->parse_volname($volname);
+    my ($vmtype) = $relative_backup_dir =~ m!^\d+/([a-z]+)-!;
+
+    return ('qemu-img', $vmtype) if $vmtype eq 'qemu';
+
+    if ($vmtype eq 'lxc') {
+	my (undef, $relative_backup_dir) = $self->{'storage-plugin'}->parse_volname($volname);
+
+	if (-e "$self->{scfg}->{path}/${relative_backup_dir}/archive.tar") {
+	    $self->{'restore-mechanisms'}->{$volname} = 'tar';
+	    return ('tar', $vmtype);
+	}
+
+	if (-e "$self->{scfg}->{path}/${relative_backup_dir}/archive.sqfs") {
+	    $self->{'restore-mechanisms'}->{$volname} = 'directory';
+	    return ('directory', $vmtype)
+	}
+
+	die "unable to find archive '$volname'\n";
+    }
+
+    die "cannot restore unexpected guest type '$vmtype'\n";
+}
+
+sub archive_get_guest_config {
+    my ($self, $volname) = @_;
+
+    my (undef, $relative_backup_dir) = $self->{'storage-plugin'}->parse_volname($volname);
+    my $filename = "$self->{scfg}->{path}/${relative_backup_dir}/guest.conf";
+
+    return file_get_contents($filename);
+}
+
+sub archive_get_firewall_config {
+    my ($self, $volname) = @_;
+
+    my (undef, $relative_backup_dir) = $self->{'storage-plugin'}->parse_volname($volname);
+    my $filename = "$self->{scfg}->{path}/${relative_backup_dir}/firewall.conf";
+
+    return if !-e $filename;
+
+    return file_get_contents($filename);
+}
+
+sub restore_vm_init {
+    my ($self, $volname) = @_;
+
+    my $res = {};
+
+    my (undef, $relative_backup_dir) = $self->{'storage-plugin'}->parse_volname($volname);
+    my $backup_dir = "$self->{scfg}->{path}/${relative_backup_dir}";
+
+    my @backup_files = glob("$backup_dir/*");
+    for my $backup_file (@backup_files) {
+	next if $backup_file !~ m!^(.*/(.*)\.qcow2)$!;
+	$backup_file = $1; # untaint
+	$res->{$2}->{size} = PVE::Storage::Plugin::file_size_info($backup_file, undef, 'qcow2');
+    }
+
+    return $res;
+}
+
+sub restore_vm_cleanup {
+    my ($self, $volname) = @_;
+
+    return; # nothing to do
+}
+
+sub restore_vm_volume_init {
+    my ($self, $volname, $device_name, $info) = @_;
+
+    my (undef, $relative_backup_dir) = $self->{'storage-plugin'}->parse_volname($volname);
+    my $image = "$self->{scfg}->{path}/${relative_backup_dir}/${device_name}.qcow2";
+    # NOTE Backing files are not allowed by Proxmox VE when restoring. The reason is that an
+    # untrusted qcow2 image can specify an arbitrary backing file and thus leak data from the host.
+    # For the sake of the directory example plugin, an NBD export is created, but this side-steps
+    # the check and would allow the attack again. An actual implementation should check that the
+    # backing file (or rather, the whole backing chain) is safe first!
+    my $nbd_node = bind_next_free_dev_nbd_node([$image]);
+    $self->{"${volname}/${device_name}"}->{'nbd-node'} = $nbd_node;
+    return {
+	'qemu-img-path' => $nbd_node,
+    };
+}
+
+sub restore_vm_volume_cleanup {
+    my ($self, $volname, $device_name, $info) = @_;
+
+    if (my $nbd_node = delete($self->{"${volname}/${device_name}"}->{'nbd-node'})) {
+	PVE::Tools::run_command(['qemu-nbd', '-d', $nbd_node]);
+    }
+
+    return;
+}
+
+my sub restore_tar_init {
+    my ($self, $volname) = @_;
+
+    my (undef, $relative_backup_dir) = $self->{'storage-plugin'}->parse_volname($volname);
+    return { 'tar-path' => "$self->{scfg}->{path}/${relative_backup_dir}/archive.tar" };
+}
+
+my sub restore_directory_init {
+    my ($self, $volname) = @_;
+
+    my (undef, $relative_backup_dir, $vmid) = $self->{'storage-plugin'}->parse_volname($volname);
+    my $archive = "$self->{scfg}->{path}/${relative_backup_dir}/archive.sqfs";
+
+    my $mount_point = "/run/backup-provider-example/${vmid}.mount";
+    make_path($mount_point);
+    die "unable to create directory $mount_point\n" if !-d $mount_point;
+
+    run_command(['mount', '-o', 'ro', $archive, $mount_point]);
+
+    return { 'archive-directory' => $mount_point };
+}
+
+my sub restore_directory_cleanup {
+    my ($self, $volname) = @_;
+
+    my (undef, undef, $vmid) = $self->{'storage-plugin'}->parse_volname($volname);
+    my $mount_point = "/run/backup-provider-example/${vmid}.mount";
+
+    run_command(['umount', $mount_point]);
+
+    return;
+}
+
+sub restore_container_init {
+    my ($self, $volname, $info) = @_;
+
+    if ($self->{'restore-mechanisms'}->{$volname} eq 'tar') {
+	return restore_tar_init($self, $volname);
+    } elsif ($self->{'restore-mechanisms'}->{$volname} eq 'directory') {
+	return restore_directory_init($self, $volname);
+    } else {
+	die "no restore mechanism set for '$volname'\n";
+    }
+}
+
+sub restore_container_cleanup {
+    my ($self, $volname, $info) = @_;
+
+    if ($self->{'restore-mechanisms'}->{$volname} eq 'tar') {
+	return; # nothing to do
+    } elsif ($self->{'restore-mechanisms'}->{$volname} eq 'directory') {
+	return restore_directory_cleanup($self, $volname);
+    } else {
+	die "no restore mechanism set for '$volname'\n";
+    }
+}
+
+1;
diff --git a/src/PVE/BackupProvider/Plugin/Makefile b/src/PVE/BackupProvider/Plugin/Makefile
index bbd7431..bedc26e 100644
--- a/src/PVE/BackupProvider/Plugin/Makefile
+++ b/src/PVE/BackupProvider/Plugin/Makefile
@@ -1,4 +1,4 @@
-SOURCES = Base.pm
+SOURCES = Base.pm DirectoryExample.pm
 
 .PHONY: install
 install:
diff --git a/src/PVE/Storage/Custom/BackupProviderDirExamplePlugin.pm b/src/PVE/Storage/Custom/BackupProviderDirExamplePlugin.pm
new file mode 100644
index 0000000..d04d9d1
--- /dev/null
+++ b/src/PVE/Storage/Custom/BackupProviderDirExamplePlugin.pm
@@ -0,0 +1,308 @@
+package PVE::Storage::Custom::BackupProviderDirExamplePlugin;
+
+use strict;
+use warnings;
+
+use File::Basename qw(basename);
+
+use PVE::BackupProvider::Plugin::DirectoryExample;
+use PVE::Tools;
+
+use base qw(PVE::Storage::Plugin);
+
+# Helpers
+
+sub get_vm_backup_mechanism {
+    my ($class, $scfg) = @_;
+
+    return $scfg->{'vm-backup-mechanism'} // properties()->{'vm-backup-mechanism'}->{'default'};
+}
+
+sub get_vm_backup_mode {
+    my ($class, $scfg) = @_;
+
+    return $scfg->{'vm-backup-mode'} // properties()->{'vm-backup-mode'}->{'default'};
+}
+
+sub get_lxc_backup_mode {
+    my ($class, $scfg) = @_;
+
+    return $scfg->{'lxc-backup-mode'} // properties()->{'lxc-backup-mode'}->{'default'};
+}
+
+# Configuration
+
+sub api {
+    return 11;
+}
+
+sub type {
+    return 'backup-provider-dir-example';
+}
+
+sub plugindata {
+    return {
+	content => [ { backup => 1, none => 1 }, { backup => 1 } ],
+	features => { 'backup-provider' => 1 },
+	'sensitive-properties' => {},
+    };
+}
+
+sub properties {
+    return {
+	'lxc-backup-mode' => {
+	    description => "How to create LXC backups. tar - create a tar archive."
+		." squashfs - create a squashfs image. Requires squashfs-tools to be installed.",
+	    type => 'string',
+	    enum => [qw(tar squashfs)],
+	    default => 'tar',
+	},
+	'vm-backup-mechanism' => {
+	    description => "Which mechanism to use for creating VM backups. nbd - access data via "
+		." NBD export. file-handle - access data via file handle.",
+	    type => 'string',
+	    enum => [qw(nbd file-handle)],
+	    default => 'file-handle',
+	},
+	'vm-backup-mode' => {
+	    description => "How to create VM backups. full - always create full backups."
+		." incremental - create incremental backups when possible, fallback to full when"
+		." necessary, e.g. VM disk's bitmap is invalid.",
+	    type => 'string',
+	    enum => [qw(full incremental)],
+	    default => 'full',
+	},
+    };
+}
+
+sub options {
+    return {
+	path => { fixed => 1 },
+	'lxc-backup-mode' => { optional => 1 },
+	'vm-backup-mechanism' => { optional => 1 },
+	'vm-backup-mode' => { optional => 1 },
+	disable => { optional => 1 },
+	nodes => { optional => 1 },
+	'prune-backups' => { optional => 1 },
+	'max-protected-backups' => { optional => 1 },
+    };
+}
+
+# Storage implementation
+
+# NOTE a proper backup storage should implement this
+sub prune_backups {
+    my ($class, $scfg, $storeid, $keep, $vmid, $type, $dryrun, $logfunc) = @_;
+
+    die "not implemented";
+}
+
+sub parse_volname {
+    my ($class, $volname) = @_;
+
+    if ($volname =~ m!^backup/((\d+)/[a-z]+-\d+)$!) {
+	my ($filename, $vmid) = ($1, $2);
+	return ('backup', $filename, $vmid);
+    }
+
+    die "unable to parse volume name '$volname'\n";
+}
+
+sub path {
+    my ($class, $scfg, $volname, $storeid, $snapname) = @_;
+
+    die "volume snapshot is not possible on backup-provider-dir-example volume" if $snapname;
+
+    my ($type, $filename, $vmid) = $class->parse_volname($volname);
+
+    return ("$scfg->{path}/${filename}", $vmid, $type);
+}
+
+sub create_base {
+    my ($class, $storeid, $scfg, $volname) = @_;
+
+    die "cannot create base image in backup-provider-dir-example storage\n";
+}
+
+sub clone_image {
+    my ($class, $scfg, $storeid, $volname, $vmid, $snap) = @_;
+
+    die "can't clone images in backup-provider-dir-example storage\n";
+}
+
+sub alloc_image {
+    my ($class, $storeid, $scfg, $vmid, $fmt, $name, $size) = @_;
+
+    die "can't allocate space in backup-provider-dir-example storage\n";
+}
+
+# NOTE a proper backup storage should implement this
+sub free_image {
+    my ($class, $storeid, $scfg, $volname, $isBase) = @_;
+
+    # if it's a backing file, it would need to be merged into the upper image first.
+
+    die "not implemented";
+}
+
+sub list_images {
+    my ($class, $storeid, $scfg, $vmid, $vollist, $cache) = @_;
+
+    my $res = [];
+
+    return $res;
+}
+
+sub list_volumes {
+    my ($class, $storeid, $scfg, $vmid, $content_types) = @_;
+
+    my $path = $scfg->{path};
+
+    my $res = [];
+    for my $type ($content_types->@*) {
+	next if $type ne 'backup';
+
+	my @guest_dirs = glob("$path/*");
+	for my $guest_dir (@guest_dirs) {
+	    next if !-d $guest_dir || $guest_dir !~ m!/(\d+)$!;
+
+	    my $backup_vmid = basename($guest_dir);
+
+	    next if defined($vmid) && $backup_vmid != $vmid;
+
+	    my @backup_dirs = glob("$guest_dir/*");
+	    for my $backup_dir (@backup_dirs) {
+		next if !-d $backup_dir || $backup_dir !~ m!/(lxc|qemu)-(\d+)$!;
+		my ($subtype, $backup_id) = ($1, $2);
+
+		my $size = 0;
+		my @backup_files = glob("$backup_dir/*");
+		$size += -s $_ for @backup_files;
+
+		push $res->@*, {
+		    volid => "$storeid:backup/${backup_vmid}/${subtype}-${backup_id}",
+		    vmid => $backup_vmid,
+		    format => "directory",
+		    ctime => $backup_id,
+		    size => $size,
+		    subtype => $subtype,
+		    content => $type,
+		    # TODO parent for incremental
+		};
+	    }
+	}
+    }
+
+    return $res;
+}
+
+sub activate_storage {
+    my ($class, $storeid, $scfg, $cache) = @_;
+
+    my $path = $scfg->{path};
+
+    my $timeout = 2;
+    if (!PVE::Tools::run_fork_with_timeout($timeout, sub {-d $path})) {
+	die "unable to activate storage '$storeid' - directory '$path' does not exist or is"
+	    ." unreachable\n";
+    }
+
+    return 1;
+}
+
+sub deactivate_storage {
+    my ($class, $storeid, $scfg, $cache) = @_;
+
+    return 1;
+}
+
+sub activate_volume {
+    my ($class, $storeid, $scfg, $volname, $snapname, $cache) = @_;
+
+    die "volume snapshot is not possible on backup-provider-dir-example volume" if $snapname;
+
+    return 1;
+}
+
+sub deactivate_volume {
+    my ($class, $storeid, $scfg, $volname, $snapname, $cache) = @_;
+
+    die "volume snapshot is not possible on backup-provider-dir-example volume" if $snapname;
+
+    return 1;
+}
+
+sub get_volume_attribute {
+    my ($class, $scfg, $storeid, $volname, $attribute) = @_;
+
+    return;
+}
+
+# NOTE a proper backup storage should implement this to support backup notes and
+# setting protected status.
+sub update_volume_attribute {
+    my ($class, $scfg, $storeid, $volname, $attribute, $value) = @_;
+
+    die "attribute '$attribute' is not supported on backup-provider-dir-example volume";
+}
+
+sub volume_size_info {
+    my ($class, $scfg, $storeid, $volname, $timeout) = @_;
+
+    my (undef, $relative_backup_dir) = $class->parse_volname($volname);
+    my ($ctime) = $relative_backup_dir =~ m/-(\d+)$/;
+    my $backup_dir = "$scfg->{path}/${relative_backup_dir}";
+
+    my $size = 0;
+    my @backup_files = glob("$backup_dir/*");
+    for my $backup_file (@backup_files) {
+	if ($backup_file =~ m!\.qcow2$!) {
+	    $size += $class->file_size_info($backup_file, undef, 'qcow2');
+	} else {
+	    $size += -s $backup_file;
+	}
+    }
+
+    my $parent; # TODO for incremental
+
+    return wantarray ? ($size, 'directory', $size, $parent, $ctime) : $size;
+}
+
+sub volume_resize {
+    my ($class, $scfg, $storeid, $volname, $size, $running) = @_;
+
+    die "volume resize is not possible on backup-provider-dir-example volume";
+}
+
+sub volume_snapshot {
+    my ($class, $scfg, $storeid, $volname, $snap) = @_;
+
+    die "volume snapshot is not possible on backup-provider-dir-example volume";
+}
+
+sub volume_snapshot_rollback {
+    my ($class, $scfg, $storeid, $volname, $snap) = @_;
+
+    die "volume snapshot rollback is not possible on backup-provider-dir-example volume";
+}
+
+sub volume_snapshot_delete {
+    my ($class, $scfg, $storeid, $volname, $snap) = @_;
+
+    die "volume snapshot delete is not possible on backup-provider-dir-example volume";
+}
+
+sub volume_has_feature {
+    my ($class, $scfg, $feature, $storeid, $volname, $snapname, $running) = @_;
+
+    return 0;
+}
+
+sub new_backup_provider {
+    my ($class, $scfg, $storeid, $bandwidth_limit, $log_function) = @_;
+
+    return PVE::BackupProvider::Plugin::DirectoryExample->new(
+	$class, $scfg, $storeid, $bandwidth_limit, $log_function);
+}
+
+1;
diff --git a/src/PVE/Storage/Custom/Makefile b/src/PVE/Storage/Custom/Makefile
new file mode 100644
index 0000000..c1e3eca
--- /dev/null
+++ b/src/PVE/Storage/Custom/Makefile
@@ -0,0 +1,5 @@
+SOURCES = BackupProviderDirExamplePlugin.pm
+
+.PHONY: install
+install:
+	for i in ${SOURCES}; do install -D -m 0644 $$i ${DESTDIR}${PERLDIR}/PVE/Storage/Custom/$$i; done
diff --git a/src/PVE/Storage/Makefile b/src/PVE/Storage/Makefile
index ce3fd68..fc0431f 100644
--- a/src/PVE/Storage/Makefile
+++ b/src/PVE/Storage/Makefile
@@ -21,4 +21,5 @@ SOURCES= \
 install:
 	make -C Common install
 	for i in ${SOURCES}; do install -D -m 0644 $$i ${DESTDIR}${PERLDIR}/PVE/Storage/$$i; done
+	make -C Custom install
 	make -C LunCmd install
-- 
2.39.5



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